Even the human eye with its remarkable ability to capture huge amounts of optical information glare, uneven illumination, and insufficient light pose substantial problems to visual assessments. Moreover, these problems are complicated by the fact that their true nature and extent are defined by the strength, position, angular orientation, and emission topology of the light source as well as the size, configuration, position, optical finish and orientation of the object being illuminated. Failure to achieve proper illumination can result in a wide range of problems ranging from increased accident rates for drivers in automobiles to unacceptably high approval rates for defective parts in manufacturing.
Today, in an attempt to avoid the high cost of human optical inspection, and also to avoid the sometimes unpredictable problems caused by fatigue, distraction and other human factors, industry has turned increasingly toward computerized inspection systems. With such systems, however, the resolution of the optical inspection apparatus is far below that of the human eye. Accordingly, factors such as glare, insufficient illumination, uneven illumination, effects caused by the optical finish of the surface, and degraded optical resolution, present serious problems.
Moreover, such inspection is typically done by a video device viewing an object through a microscope. In a microscope, the objective housing is positioned over the object to be viewed. Light must be caused to fall on the object in a multitude of directions in order to illuminate all features of the object. However, the space available for placing a light source in a microscope is very limited. The problem is further complicated because much of that space is taken up by the object and the optical train (centered on the optical axis of the microscope) of the microscope which must be kept near the object in order to keep the object visible through the eye piece of the microscope.
The classical approach to this problem is to place a so-called ring illuminator just below the microscope objective. The ring illuminator is comprised of a circular ring like mechanical member which supports a plurality of light sources disposed around the circular periphery of the illuminator. The light sources have a pattern which shed the overwhelming majority of their output light within an angular subtense having a range of between, 1) directly and perpendicularly downward in the direction of the plane of the object being viewed and 2) downward through a ray path oriented between a point on the circular periphery of the illuminator and a point on the optical axis of the focal plane at a specified distance below the plane defined by the ring illuminator.
The advantage of the ring illuminator is that its center is open, thus allowing the microscope objective to collect light rays directly below the objective, and at the same time, light is provided from a plurality of points along the periphery and aimed, to a large extent, at the space below the ring illuminator.
While a ring illuminator is an effective way of shedding light on object without blocking the optical train or optical path of a microscope, it tends to suffer from all of the illumination problems discussed above. Indeed, because ring illuminators generally comprise a plurality of point sources, the above problems can be particularly egregious.
In particular, ring illuminators generally have the defect, as in the case of the illumination of the top of a cylindrical object, of presenting the image of two bright stripes, thereby tending to conceal the surface area of interest. This is particularly true of specular (highly reflective) surfaces.
The demands put on illumination systems by so-called "machine vision systems" have spawned a wide variety of devices. Generally, such systems followed the lead of the ring illuminator, comprising sources which are substantially symmetrical along orthogonal axes. See, for example, the system disclosed in U.S. Pat. No. 5,461,417.
The approach in many machine vision illumination systems is to use a light diffusing plate in conjunction with a beamsplitter to achieve the diffuse illumination required in certain applications. In particular, light is output by a diffuse light source, such as a translucent or an opal glass plate which is illuminated from behind, and this light is caused to fall on the beamsplitter or half-silvered mirror which is oriented to reflect the light onto the workpiece under observation. At the same time, because the beamsplitter is semi-transparent, and positioned in the optical train of the microscope, the beamsplitter allows light to pass from the workpiece into the microscope objective which then collects and focuses the light onto the detector of the machine vision system. While such systems do address some of the aforementioned problems, it has been discovered in accordance with present invention, that machine vision illumination systems suffer from the relatively inefficient use of light. In addition, there is a tendency to obscure many surface details. Indeed, the tendency to destroy surface details is useful if one wishes to inspect other features, such as reflectivity or the like.
Such problems are, however, particularly serious in the case of objects having a cylindrical configuration. By "cylindrical configuration", in this specification, the applicant wishes to embrace the general definition of a cylindrical surface, that is to say a surface which is defined by the movement of a line along a curve, with the line maintaining an orientation parallel to its original position, as well as surfaces which, while not strictly cylindrical in shape, share optical characteristics and illumination problems in connection with cylindrical surfaces. Thus these surfaces include the surface of a right cylindrical cylinder, as well as a number of other surfaces which could be formed by the curling of a planar sheet of material, such as paper.
When the cylindrical object is, for example, made of metal and thus has a relatively shiny optical finish, the above problems are substantially complicated. More particularly, if the object is a right cylindrically shaped object, and is formed of a metal mesh or expanded metal material, the large number of elements having orientation substantially parallel to the axis of the cylinder will present a plurality of reflections, under illumination from conventional light sources, whose intensity will tend to over power the ability of the optical sensor to detect the edge of the object adjacent to the glarish reflection.